Sulfide coating



Patented July 7, 1 953 ENT 4,oF-'FICE sULFIDE ooATING John E. Baxter,` Maywood, Calif., assgn'or to Parker Rust Proof Company, Detroit, Mich., a Y

corporation of Michigan l. f

original application Januaryiz, 195o, serial No. y p, 138,251. Divided and this application April 18` 1952, seria1N0.2s2,9ss f 4 claims. (01.'148-624) 1 This invention concerns a composition and methodfor the forming of suldecoating on a steel surface, particularly on corrosion resistant and stainless steel surfaces, y y

This application is a division of priorcopending application, Serial No. 138,251, led January f It is known that acid solutions of sulfites and certain other sulfur compounds can be used to attack ythe surface of a ferrous metal to form a sulfide coating thereon. AThese Vcoatings may serve various functions. For example, in some cases they are used as a paint base and in some cases they areused as a protective coating for the metal when the latter is subjected to wear or to extensive plastic deformation as in drawing and the like. The coatings, because of their insolubility in acid solutions, are superior Vto phosphate and oxalate coatings in those uses which require contact of the coating with oils of high acidity.

In order to form such acca-ting, it 1s necessary that the solution attack the surface of the metal and form a salt of the metal. kIt has been pointed out, however, that the acid should not be sufficiently strong to cause excessive pickling` of the metal since such pickling action, if too energetic, will prevent the forming of a coating. Therefore, -it has been the practice to limit the acidity of the solution and to increase the rate of attack of the solution on the metal to the point desired by a suitable increase in the solutio temperature. e

The problem of securing both adequate and a controllable attack of the acid on the metal sur- A face has been accentuated in the case of stainless steels which are normallyrquite resistant to acid attack. The forming of sulfide coatings on such normally resistant metal surfaces is usually carried out at temperatures near the boiling point of water, and, at best, in the range of 170 lik-200 F'. Therefore, the process has been subject to al1-the disadvantages of nigh temperature operation; particularly the high vapor pressure of the treating solution and the diiiculty of handling acid solutions at suchvtemperatures. n It is an object of this invention to provide a solution and a method for the forming of a suli'lde coating on the surface of a steel object, particularly on a corrosion resistant or on a stainless steel object. y

A further object is to provide a strongly acid composition for the treatment of a steel surface to form a sulfide coating thereon while avoiding the excessive pickling` eifect normally encountered vl/ith-suchcoinpositions.A ,v v f 'It' is a further object to form asulde coating on a steel surface, particularly corrosion resistant-and stainless steel surfaces, at temperatures much lower than those used heretofore.

I have now found that the foregoing and related objects can be secured by subjecting the surface of the steel to the action of an acidic aqueous solution comprising as essential ingredients the sulde ion or compounds capable of yielding the sulfide ion at said steel surface; the thiocyanate ion; and va .proportion of lan acid suilicient to provide a'pH below about 1.1.

Preferred compositions include Lthe suliite ion; but sulfide, thiosulfate, reducible organic sulfurcontaining ions, etc., may be used. All such ions, other than the'sulde ion itself, formsulde ions at the metal surface through reduction by the metal. f

v,The drawing illustrates graphically, the effect of the addition of thiocyanate in lowering the temperature required to produce a satisfactory coating on an 18-8 stainless steel in an arbii curve was determined by a large number'of individual determinations, and istypical of those obtained with a kvariety of acids and a variety of sulfide ion producing materials.

I have found that two limitations, that is, the

degree of acidity-and the-presence of thiocyanate ion are necessary to the obtaining of a successful result. If the pH of the solution is higher than about 1.1 the addition of the anion has little advantage.

a marked effect; especially in the lowering of the temperature required for the yformation of an effective coating in a reasonably short time. For example, as littlekas about .05% thiocyanate s has a marked effect on the temperature required wherer the acidity is less vthan a, pH of 1.1 as may be seen in the drawing.

The lower limit of pH is not critical and depends somewhat on the particular acid chosen,

Where, however, the pH of the solution is less than about 1.1, even very small percentages of the thiocyanate ion haveA the concentration of the thiocyanate ion, the degree of pickling action produced, the particular steel being coated and the degree of pickling which can be tolerated for the type of coating desired. However, it may be stated as a general rule that the pH range to be used ls from 0.3 to 1.l. No particular advantage is gained by operating at the extreme limits of acid concentration since the entire advantage of the invention can be obtained when the pH is less than.1.1 and substantially above 0.5. If, however, it be desired to operate at the lowest possiblepH. it is a matter of a few simple tests to establish the lower limit permissible with the particular steel and the thiocyanate ion. Y

The sulfite ion concentration is normally maintained at about 0.3 to 1.3% but this proportion has been varied over relatively wide limits. It is preferred that the thiocyanate ion be maintained above about 1% in order to secure the maximum benefits from the invention, but some advantage can be'gained with very minor percentages. Highly concentrated solutions, up to saturation, of the various ions can be used but little advantage is gained above 1.3% sulfite or about 5% of thiocyanate ion.

The particular acid selected is-not important except that it is strong enough to pr-ovide the required pH. The `mineral acids are both stronger and inexpensive yas compared with other acids, e. g benzene sulfonic land trichloracetic, and hence are preferred. sulfuric, hydrochloric and nitric acids are especially preferred. AAcids whichnare not sufficiently strong to give the desired DH may ibe used if a suitable proportion of -a stronger-acid isV used in the same composition. Similarly the -source of sulfite, thioc anate, etc., is not important provided lthe cation provided with said ion be compatible with the solution; that is, not precipitate out an essential ingredientor have ani7 other adverse veifect on the solution or the-coating.

The metals treated comprise the entire category of steels including ,mild steels, corrosion resistant, andstainless steels and, .as pointed out previonslmlhe invention has rparticular advantage Vin treating stainless steels `beca-.use .of the avoidance of the high temperatures normally required yintheir coating. The coatings produced are :salts lof the `metals treated and vary somewhat #withxliiferent alloys. Gertain .of the metals encountered `in alloy steels have :a :tendency to form an oxide rather than a .sulfide and, therefore, Ithe average coating contains some oxide. Nickel appears to `be particularly active toward the compositions of the invention :and nickel steels usually produce a high percentage of nickel sulfide in the coating.

The following examples are given to more clearly illustrate the method and solutions 1.0i the invention:

' vExample A1 A solution was made -up vcontaining 10 Vgrams of sodium suliite vlper gallon of -water and con taining 20 cc.-of` 95% sulfuric acid, the solution having `a pH of `'1.11. -A temperature of '200 was required to produce a coating `on the stainless Vsteel iin vthree minutes.

Esample 2 A 'series of solutions containing .5% sulfuric acid and 0.32% vsulfite was 4made up. AEach of the solutions contained successively the following thiocyanate percentages: 0.0, 0.1, 0.3, 0.5 and 5.0%. In the solution containing no thiocyanate, a coating was produced in three minutes at 157 F. on stainless steels. In the remainingsolutions containing .1 to 5% thiocyanate, a satisfactory coating was obtained in three minutes at 125, 105 and 80 F., respectively.

Although I do not wish to be limited by theory, `I believe that the thiocyanate ion has an activating influence on the acid attack on the metal. This permits a suitable and controlled attack at a lowertemperature for the purpose of forming a coating on the metal. It appears also that the activation does not significantly increase the pickling effect, but in some cases, may reduce this effect when full advantage is taken of the-lowered coating temperature. As indicated previously, the invention permits of a more rapid coating of stainless and corrosion resistant steels and is particularly advantageous in this respect.

What is claimed is:

l. A ,composition for forming a sulfide coating on a steel surface which consists essentially of a member of the group consisting of the sulfide ion and ions which form the sulfide ion through reduction on the steel surface and in a proportion equivalent to about .3 to 1.3% Suliite ion, the thiocyanate ion in a proportion of from .05% up to and including saturation, and an acid selected from the group consisting of sulfuric, hydrochloric and nitric in a proportion suiiicient to provide a pH in the range of about .3 to 1.1.

2. A composition for forming a sulfide coating on a steel surface which consists essentially of a member of the group consisting oi the sulfide ion and ions which forni the sulfide ion through reduction on the steel surface and in a proportion equivalent to about .3% to 1.3% su'lfite ion, the `thiocyanate ion in a proportion of from 1% to 5%, and an ,acid selected from the group consisting of sulfuric, hydrochloric and nitric in a proportion sufficient 4to provide a pH in the range of about .3 `to 1.1.

The vmethod of forming Aa sulfide coating on a' steel surface which comprises contacting the surface with a solution consisting `essentially of the sulfide ion and ions which form the sulfide ion through reduction on the steel surface and in a proportion equivalent to about .3% rto :3% sulfite ion, the thiocyanate ion in a proportion of Ifrom 05% up to 'and including saturation, and 'an acid selected lfrom the `group consisting of sulfuric, vhydrochloric and nitric in a proportion .sufficient to provide a pH -in the range Vof about ,.3 to 1.1.

'4. The method of forming -a lsulfide coating on a steel Ysurface `which comprises contacting the surface with a solution vconsisting essentially of ithe sulfide ion and ions which form the sul.- flde ion through reduction on the steel .surface and yin ,-a proportion equivalent -to about .3% to 1.3% sulfite ion, the thiocyanate ion in a proportion of from 1% to 5%, an an acid selected from Ythe group consisting of sulfuric, hydrochloric and vnitric in a proportion sufficient `to provide a-pH in the range of about .3 to 1,1.

JOHN BAXTER.

No references cited. 

1. A COMPOSITION FOR FORMING A SULFIDE COATING ON A STEEL SURFACE WHICH CONSISTS ESSENTIALLY OF A MEMBER OF THE GROUP CONSISTING OF THE SULFIDE ION AND IONS WHICH FORM THE SULFIDE ION THROUGH REDUCTION ON THE STEEL SURFACE AND IN A PROPORTION EQUIVALENT TO ABOUT .3 TO 1.3% SULFITE ION, THE THIOCYANATE ION IN A PROPORTION OF FROM .05% UP TO AND INCLUDING SATURATION, AND AN ACID SELECTED FROM THE GROUP CONSISTING OF SULFURIC, HYDROCHLORIC AND NITRIC IN A PROPORTION SUFFICIENT TO PROVIDE A PH IN THE RANGE OF ABOUT .3 TO 1.1. 